It is a common practice to treat subterranean formations to increase the gross permeability or conductivity of such formations by procedures which are identified generally as fracturing processes. For example, it is a conventional practice to hydraulically fracture a well in order to produce one or more cracks or "fractures" in the surrounding formation by mechanical breakdown of the formation. Fracturing may be carried out in wells which are completed in subterranean formations for virtually any purpose. The usual candidates for fracturing, or other stimulation procedures, are production wells completed in oil and/or gas containing formations. However, injection wells used in secondary or tertiary recovery operations, for example, for the injection of water or gas, may also be fractured in order to facilitate the injection of fluids into such subterranean formations.
Hydraulic fracturing is accomplished by injecting a hydraulic fracturing fluid into the well and imposing sufficient pressure on the fracturing fluid to cause the formation to break down with the attendant production of one or more fractures. The fracture or fractures may be horizontal or vertical, with the latter usually predominating, and with the tendency toward vertical fracture orientation increasing with the depth of the formation being fractured. Usually a gel, an emulsion or a foam, having a proppant such as sand or other particulate material suspended therein is introduced into the fracture. The proppant is deposited in the fracture and functions to hold the fracture open after the pressure is released and the fracturing fluid is withdrawn back into the well. The fracturing fluid has a sufficiently high viscosity to penetrate into the formation to realize fracturing and to retain the proppant in suspension or at least to reduce the tendency of the proppant to settle out of the fracturing fluid. Generally, a gelation agent and/or an emulsifier is used to gel or emulsify the fracturing fluid to provide the high viscosity needed to realize the maximum benefits from the fracturing process.
After the high viscosity fracturing fluid has been pumped into the formation and the fracturing of the formation has been obtained, it is desirable to remove the fluid from the formation to allow hydrocarbon production through the new fractures. Generally, the removal of the highly viscous fracturing fluid is realized by "breaking" the gel or emulsion or, in other words, by converting the fracturing fluid into a low viscosity fluid. Breaking the gelled or emulsified fracturing fluid has commonly been obtained by adding a "breaker," that is, a viscosity-reducing agent, to the subterranean formation at the desired time. However, known techniques can be unreliable and at times result in incomplete breaking of the fluid and/or premature breaking of the fluid before the fracturing process is complete. Premature breaking can cause a decrease in the number of fractures obtained and thus, the amount of hydrocarbon recovery.
There have been several proposed methods for the breaking of fracturing fluids which were aimed at eliminating the above problems. For example, methods for breaking well-treating fluids containing guar gum as a viscosity-imparting additive are known in the art. One such method includes the use of enzyme preparations. The use of enzyme preparations is effective in bringing about a rapid breaking of the fracturing fluid, but such enzyme use possesses a number of disadvantages. The enzyme is generally mixed with the guar gum in the dry state. This mixture is fairly stable in storage, but as soon as the preparation is hydrated, enzyme action commences and the guar gum is hydrolyzed. The guar gum is hydrated at the surface before it is injected into the well. As long as two hours may be required to hydrate the gum, and during that time a considerable degree of hydrolysis will take place. To compensate for this loss, additional quantities of guar gum must be used initially. See U.S. Pat. No. 3,167,510.
U.S. Pat. No. 3,684,710 discloses a combination of two enzymes from different microbiological sources with the same substrate specificity, but with different characteristics with respect to pH-activity and pH-stability. The patent teaches that due to the extreme sensitivity of enzymes to changes in pH it has been found that the usefulness of many enzyme systems in practical commercial processes is severely limited or else the systems are subject to less than optimum usage. The patent teaches the use of two enzymes, one being active from pH of 5-7.5 and one being active from pH of 2.5-5 to break fracturing fluids. The patent teaches that the natural pH of the earth, which is generally below 9, serves to activate the enzymes and the enzymes exhibit activity over the pH range of 2.5-7.5. Thus, the system is dependent on the pH of the subterranean formation and does not provide a system wherein the pH is controlled and thus the combination of two enzymes is necessary due to the fact that the pH of different subterranean formations is not the same but is only generally below 9.0.
U.S. Pat. No. 4,506,734 also provides a method for reducing the viscosity and the resulting residue of an aqueous or oil-based fluid introduced into a subterranean formation by introducing a viscosity-reducing enzyme contained within hollow or porous, crushable and fragile beads along with a fluid, such as a hydraulic fracturing fluid, under pressure into the subterranean formation. When the fracturing fluid passes or leaks off into the formation or the fluid is removed by back flowing, any resulting fractures in the subterranean formation close and crush the beads. The crushing of the beads then releases the enzyme into the fluid. This process is dependent upon the pressure of the formation to obtain release of the enzyme breaker and is thus, subject to varying results dependent upon the formation and its closure rate.
Enzyme breakers for fracturing fluids also suffer from another disadvantage. Enzymes are generally very pH sensitive in that they are not active except at certain pH levels. Most often, the pH of the fracturing fluid does not correspond to the pH at which an enzyme capable of breaking that fluid is active. Also, if the pH of the fluid is at a level wherein an enzyme capable of breaking the fluid is active, then the fluid may be broken prematurely and thus the efficiency of the fracturing process is compromised.
An additional disadvantage of using enzyme preparations includes enzymatic hydrolysis which becomes extremely rapid at temperatures around 140.degree. F. This is a temperature which is commonly found in wells and when it is encountered, the viscosity of the fluid may be reduced by the enzyme below that viscosity required for proppant suspension before the proppant can be forced into the formation and is thus undesirable.
Reducing the viscosity of well-treating fluids in order to facilitate their removal from the producing formation also includes natural breaking done by bacterial degradation or by subjection to high temperatures. These methods suffer from the excessive length of time required to complete the breaking, such as several days or longer.
There remains a need for a method for controlled breaking of fracturing fluids which is more economical and provides not only controlled release of the breaker, but reduces damage to the formation and facilitates well clean-up.